Providing respiratory or ventilation therapy to a patient is a well known medical procedure intended to aid in the stabilization and recovery of the patient. This therapy is often provided in surgical and critical care situations. Typically, an endotracheal breathing tube is inserted into the patient's mouth and then into their trachea so that the distal end of the breathing tube is disposed in the trachea before it branches into the bronchi that lead to the lungs. The proximal end of the breathing tube is usually connected to an airway tube that leads to a controllable gas supply delivery system--the ventilator. The ventilation therapy gas may be pure oxygen, atmospheric air mixed with the oxygen, and/or medication mixed with either oxygen or oxygen and air. The breathing and airway tubes thus serve as primary intake and exhaust pathways for inhalation and exhalation gases entering and leaving, respectively, the patient's body through the lungs. In this way the attending physician or clinician can closely monitor the functioning of the patient's respiratory system, particularly, and the overall health and well-being of the patient, generally.
When a balloon cuff is used the patient is dependent upon the ventilation apparatus to be able to breathe. Thus, if the ventilator responds slowly when the patient begins to inhale, the patient will be forced to breathe initially on his own against the ventilator since no air mixture is being supplied thereby; that is, there is no ventilation therapy gas flow from the ventilator. Similarly, if the ventilator responds slowly when the patient begins to exhale, the patient may be forcing exhalation gases from his lungs against the supplied ventilation therapy gas pressure. In either situation, this can be extremely tiring to a patient who is already in a weakened condition. The patient is forced to generate and expend a significant amount of energy working against the ventilator. This phenomenon is known as patient/ventilator asynchrony.
Some of the presently available ventilator models attempt to diminish the effects of patient/ventilator asynchrony by relying upon an abdominal triggering sensor to facilitate the supply and exhaust of gas to and from the patient's body. These models rely upon a sensor disposed near the patient's abdomen to sense abdominal movements supposedly indicative of inhalation and exhalations of the patient, and to begin and terminate ventilation therapy gas flow in response to the sensed abdominal movements. Other known ventilators may rely upon a prediction control method and apparatus that utilizes a learning function in order for the ventilator to "know" when to supply air to the patient. Neither type of ventilator is completely successful at reducing the patient fatigue caused by patient/ventilator asynchrony.
In addition to the fatigue induced by patient/ventilator asynchrony, damage known as barotrauma can occur to the lungs through over pressurization of the ventilation therapy gas supplied to the patient through the breathing tube. Because of this potential for lung damage, the gas pressure is usually monitored, often at a location within the breathing apparatus outside the patient's body. Known pressure sensing devices are installed in the ventilator and pressures in the airway and breathing tube are transported to the ventilator for measurement of the pressures at the proximal end of the breathing tube. These known pressure sensing devices provide a feedback signal of the pressure to a pressure regulator within the ventilator so that the pressure of the gas supplied through the breathing tube can be regulated.
Each of the present airway pressure sensing technologies suffers from several deficiencies. Among them is the inaccuracies between pressures registered at the ventilator and actual pressures within the lungs. These inaccuracies can lead to the aforementioned possibility of barotrauma to the alveoli of the lungs, particularly those of infants, due to the ventilation therapy gas being supplied to the patient at excessive pressures. These inaccuracies are partly due to inherent pressure gradients within the breathing circuit between the lungs and the location of the pressure sensor and partly to delays in the pressure sensing and response times of the ventilation equipment to excessive pressures. Because known technologies rely upon the transmission of changes in the gas pressure in the lungs to a location outside the lungs, the transmission occurring as a pressure wave being propagated down a tube, the speed of the propagation of that pressure wave determines the speed at which the ventilating equipment can respond. Additionally, because of those inherent delays in the sensing technologies currently being used, the energy required of the patient during respiration is greater than it would be if the signaling were more rapid, that is, if the aforementioned breathing circuit signal delays did not exist, since the patient is required to inhale and exhale against the ventilator--the patient/ventilator asynchrony problem. Finally, when the patient is forced to work against the ventilator, the effectiveness of the ventilation therapy is reduced because the percentage of the lung alveoli being ventilated is reduced from the optimum.
It would be desirable to have a patient ventilator that provided in situ sensing of the ventilated gas pressure as close as possible to the patient's lungs, that reduced or eliminated breathing circuit signal filtering delays, that allowed for rapid breath activation and triggering of the ventilation apparatus, that reduced the work of the patient in breathing, and that increased the effective alveolar ventilating volume.